Section 12. If it is averred in the petition that there are necessary or proper respondents whose names are unknown to the petitioner, they may be described generally, as heirs or devisees of a person deceased, as persons claiming under certain persons named, as the owners of certain land, or otherwise, and if it is so averred that the true name of a respondent cannot be ascertained, he shall be described as accurately as practicable. If it is so averred that there are classes of necessary or proper respondents whom it is impracticable and unnecessary to name and to serve with process individually, they may be described generally. The court may, if in its opinion the petitioner can and should do so, require him by amendment to name or describe respondents more particularly.